Aircraft typically use aluminum skin panels to make up the exterior of the fuselage. Each skin panel typically has thick portions and thin portions. To form the thin portions, a gage reduction operation is performed. The gage reduction typically used is chemical milling. Chemical milling uses a caustic solution in a dip tank to remove a specific pattern and thickness of aluminum from the interior surface of the skin. Then, a phosphoric anodized acid operation is conducted at another dip tank to prepare the interior surface for painting. A protective spray maskant is applied to both the interior and exterior of the panel to provide these operations. The areas of the skin panel that require chemical milling and phosphoric anodized acid exposure are not applied with maskant or have the maskant removed in the specific engineering pattern required. The protective spray maskant is not a perfectly sealed surface and suffers from voids, air bubbles, and is occasionally damaged during the handling of the large skin panels. If not repaired, the imperfections in the protective skin maskant will allow the caustics and acids in the dip tanks to attack the skin panel. Such panels must be scrapped. Each panel is on the order of thousands of dollars.
To uncover any imperfections in the protective maskant, a spark checking operation is typically performed. A spark checker uses a high-voltage electrical current to uncover voids, damage and thin spots in the protective maskant. The operator of a spark check guides a wand across the maskant surface. Any imperfections allow an electrical current to arc through to the aluminum skin panel. These areas are then easily identified and may then be repaired.
Spark checker devices must produce a proper voltage to generate the arc. Most spark checkers are battery operated and rely on a coil to achieve the proper voltage at the wand. As the battery runs down, it is difficult for the operator to be sure if enough voltage is being generated to allow the unit to function properly. One type of device is known as a “holiday detector.” Such devices have a digital voltmeter that is used to read the voltage being generated. However, the digital voltmeter has readings that jump erratically and thus, it is difficult to use. The voltmeter is typically connected to the battery and not to the wand itself. Therefore, the operator does not know if the coil is functioning properly or if the cord connections to the wand are secure. Other spark check devices do not provide any indication of voltage on the system. Such systems have been blamed for missing maskant imperfections. One problem with providing a high-voltage voltmeter is that typically the readings are erratic and difficult to monitor. Voltmeters are also relatively expensive.
It would therefore be desirable to provide a reliable and inexpensive way in which to determine if a sufficient voltage to check for maskant defects is being generated by a spark checker.